Disappointed federal officials announced Thursday that a nearly 4-year-old Mississippi girl, thought to have been cured of HIV, has detectable levels of the virus.

The child known as the "Mississippi baby" — whose apparent cure was reported in The New England Journal of Medicine last fall — has had the virus return after more than two years off anti-retroviral therapy, according to specialists involved in the case who spoke in a Thursday news briefing.

"Certainly, this is a disappointing turn of events for this young child, the medical staff involved in the child's care and the HIV/AIDS research community," said Anthony Fauci, director of the National Institute of Allergy and Infection Diseases, at the briefing.

The development "reminds us that we still have much more to learn about the intricacies of HIV infection and where the virus hides in the body," Fauci said in a statement. "The NIH remains committed to moving forward with research on a cure for HIV infection."

Although doctors heralded the girl's case as the first cure of HIV in a child last year, they also held their breath, hoping her disease wouldn't come raging back. In a routine clinical visit this month, the child was found with detectable HIV levels. A repeat test confirmed those findings.

"Ever since we discovered this case in 2012, we knew this could happen," said Hannah Gay, a pediatric HIV specialist at the University of Mississippi Medical Center in Jackson who treated the child.

Even so, the girl's relapse felt like a "punch in the gut," Gay says.

Doctors have resumed giving the child anti-HIV medications, and she is responding well. Because people with HIV are typically treated for the rest of their lives, Gay says, the girl is expected to take medication "for a long time."

Fauci says doctors hope to learn whether it's ever possible to stop anti-HIV medication. He said patients and their families should never assume it's safe to simply stop anti-HIV treatment, which could allow the virus to return.

"The fact that this child was able to remain off anti-retroviral treatment for two years and maintain quiescent virus for that length of time is unprecedented," said Deborah Persaud, professor of infectious diseases at the John Hopkins Children's Center in Baltimore and one of the two pediatric HIV experts involved in the ongoing analysis of the case. "Typically, when treatment is stopped, HIV levels rebound within weeks, not years."

Last year, doctors said the landmark finding could help scientists better understand the nature of HIV and could help countless HIV-positive babies in developing countries.

From the beginning, experts noted that the girl's story was unique — involving a string of unusual events — and wouldn't immediately lead to a cure for the 34 million people living with HIV worldwide.

The baby contracted HIV at birth.

The girl's mother tested positive for HIV when she arrived at the hospital to give birth. The mother hadn't had any prenatal care, including anti-HIV therapy. The baby was born too quickly for doctors to begin any therapy before delivery.

Because doctors suspected the baby would be infected, they began administering anti-AIDS therapy the day after birth.

It's rare for babies in the USA to be born with HIV.

A health educator massages a drop of blood from a man's finger while conducting an HIV test.(Photo: Chip Somodevilla, Getty Images)

Doctors routinely treat HIV-positive pregnant women with anti-retroviral medications and give preventive treatment to babies for the first six weeks of life.

This regimen has reduced mother-to-child transmission from 25% to 30% — in the days before anti-retroviral therapy — to less than 2% today, said Gay, in an interview with USA TODAY in 2013.

Normally, doctors would keep an HIV-positive baby like this on medications continuously, for fear that the virus would come raging back, Gay said.

In another unusual twist, however, the girl and mother disappeared when she was about 18 months old, Gay said. They missed several appointments, leading doctors to seek help from social services to track them down.

The mother and toddler finally returned to the hospital five months later, having taken no anti-AIDS medication during that time.

While most HIV-positive mothers do their best to give children recommended medications, a small number drop out of care, Gay said last year. "It's more common than we would like," she said, noting that many of those infected with HIV suffer from poverty, homelessness and other problems that can interfere with care.

Doctors fully expected to find high levels of HIV, Gay said.

They were surprised to learn that the child appeared HIV-free, with virus levels "undetectable," according to the 2013 study, co-led by researchers at Johns Hopkins' Children Center in Baltimore.

Fauci says that doctors had hoped that giving anti-retroviral drugs so early prevented the AIDS virus from hiding in her white blood cells, which can serve as "reservoirs" of infection. These reservoirs of hidden cells can cause the disease to come back if patients stop their medications.

Fauci said last year that the child's case offered support to something scientists have long believed: that a cure is possible "if you can get somebody treated before the reservoir of virus forms in the body, and before the immune system has been damaged by months or years of viral replication."

Most adults, however, are infected for many months or years before even being diagnosed, Fauci said.

Yet cures remain rare and exceptional.

The first man cured of HIV, Timothy Brown of San Francisco, also was diagnosed with leukemia. In 2006, he received a bone-marrow transplant while living in Berlin. That's a harrowing procedure in which doctors eradicate a patient's own immune system, then repopulate it with immune cells from a donor.

In Brown's case, the donor carried a genetic mutation that protects people from HIV. He was able to stop anti-AIDS therapy and remains disease-free.

Doctors would never put the millions of people currently infected with HIV through such a grueling and expensive therapy, which can cost upward of $100,000 per patient.

In the early days of AIDS, doctors sometimes reported seeing infected babies who appeared to spontaneously clear the infection on their own. Those reports all came before doctors had access to new, incredibly sensitive tests, which can detect very small levels of virus, suggesting the cases may have been misdiagnoses, Gay said last year.